1. Field of the Invention
The invention relates to a method of manufacturing a catalyst comprising argillaceous minerals and silica gel for the hydration of olefins to alcohols.
2. Discussion of the Background
It is known that olefins can be converted to alcohols in the gas phase at elevated pressures, using steam. Such methods have become particularly important in industry for manufacturing ethanol and isopropanol from ethylene and propylene, respectively. These alcohols are synthesized in the presence of catalysts, namely usually phosphoric acid, provided on a support.
Known supports are either based on pure silicic acid, e.g., kieselguhr or kieselgel (U.S. Pat. No. 2,579,601) or on silicic acid with a greater or lesser amount of alumina, e.g., calcined diatomaceous earth, wherein the structure of the material is held together by clay or clay-containing material (Ger. Pat. No. 2,722,616; U.S. Pat. No. 3,311,568).
With supports based on pure silicic acid, there are problems of the strength of the material over long service times. The alumina-containing materials have much better mechanical strength, but when the alumina content is too high, aluminum oxide is leached out by the phosphoric acid.
In Ger. Pat. No. 1,156,772 a method is described of manufacturing an alumina-containing support for the phosphoric acid used as a catalyst in olefin hydration, wherein formed catalyst bodies comprised of mineral aluminosilicate are treated with mineral acid such that the aluminum oxide content is preferably reduced to between 1 and 5 wt. %. This material in general has the required mechanical strength, along with a small enough residual aluminum oxide content to avoid the above-described leaching. On the other hand, when commercially available catalyst bodies are employed for manufacturing the catalyst supports for olefin hydration, it is observed that catalyst activities vary widely if the raw material is not selected carefully in advance.
Supports for phosphoric acid have also been developed which are based on large-pore kieselgels and which have high hydration activity and ample mechanical s strength (Ger. Pat. Nos. 2,625,705 and 2,719,055). However, these supports, based on amorphous silicic acid, have the disadvantage that upon extended service time under the conditions of the hydration reaction the amorphous silicic acid partially crystallizes, resulting in a substantial and irreversible reduction in the specific surface and thereby in the catalytic activity, as well as reduced mechanical strength.
Another disadvantage of all previously employed hydration catalysts comprising phosphoric acid on a siliceous (silicate) support is the slow decrease in activity due to removal of phosphoric acid, which must be neutralized with alkali when operating in a continuous mode, in order to avoid corrosive effects of the acidified raw alcohol on downstream equipment.
By continuous injection of phosphoric acid in an amount corresponding to the amount of phosphoric acid removed, it is possible to substantially prevent the continuing loss of activity (Ger. Pat. No. 2,658,946), and thereby greatly extend catalyst life. However, this imposes corresponding requirements on the life of the carrier itself, so that one may use a carrier in which crystallization occurs, under the reaction conditions, accompanied by irreversible reduction of the catalytic activity, and in which the mechanical strength decreases with time.
As shown in Ger. Pat. No. 2,908,491, the use of argillaceous minerals results in a support for a hydration catalyst providing constantly high catalytic activity, if, by careful choice of raw materials, one ensures that the material comprises montmorillonite to a high degree. The montmorillonite leads to high surface and large sorption volume after the forming, leaching and impregnating.
The thus manufactured catalysts and supports comprising montmorillonite-containing clay have greater activity than catalysts and supports manufactured from formed catalyst bodies based on mineral aluminosilicate of different origin, i.e. per hour and per liter of catalyst bed one obtains about 105-110 g ethanol, or about 300 g isopropanol. However the only way to maintain this high activity over an extended time is to continually add phosphoric acid to replace that which was been removed. The amount of such acid for ethanol manufacture is about 0.07 g per hour per liter catalyst bed, and for isopropanol manufacture about 0.01 g per hour per liter catalyst bed. One must still neutralize the removed acid with alkali. The mechanical strength of the catalysts is on the order of 70-90 Newtonsphere, which is sufficient for charging typical reactors.
Another improvement of the catalyst properties was achieved according to Ger. Pat. No. 2,929,919. With a catalyst produced according to Ger. Pat. No. 2,908,491, according to Ger. Pat. No. 2,929,919 there is also added during the process an oxide or a plurality of oxides of elements of group VIa of the periodic table, in the amount of 5-15 wt. % based on the total dry weight. The improvements obtained include increase of mechanical strength to 110-130 N/sphere, increase of catalytic activity to yield about 130 g ethanol and about 350 g isopropanol per hour per liter catalyst bed, and reduction of phosphoric acid removal to about 0.035 g per hour per liter catalyst bed for ethanol manufacture, and to about 0.005 g per hour per liter catalyst bed for isopropanol manufacture. It is seen that this reduction of phosphoric acid removal is approximately one half, in both cases.
Thus a catalyst with adequate service life and extended retention of mechanical strength was devised, with the phosphoric acid removal having been reduced to a satisfactory level. At this point, the only way in which the primarily silica-gel based supports were superior to montmorillonite-based supports was in their initial activity. For example, according to Ger. Pat. No. 2,722,616, up to 144 g ethanol was produced per hour per liter catalyst bed, but only 115 g isopropanol per hour per liter catalyst bed. In U.S. Pat. No. 3,311,568, a substantially higher catalyst activity was described, with 240 g ethanol being produced per hour per liter catalyst bed. Here phosphoric acid was added gradually over all 24 hours each day, and the activity was observed over only 1,500 hour, thus about 2 months. It is reported that the strength increased with increased proportion of bentonite. Bentonite was mixed into the diatomaceous earth in the amount of 3-5% prior to the sintering, according to this reference.